Method of processing a mixture of liquefied gases



July 19, 1966 A. HARMENS METHOD OF PROCESSING A MIXTURE OF LIQUEFIED GASES Filed 001;. 21, 1963 2 Sheets-Sheet 1 fi |5 Leon Gus 5 9 '4 I I5 I? F, G. Condenser I3 8 Liquid 40 Compressors J I? E Leon Gus-260F. 5 37 l L- 1/ \I "j/ 3 Fractionation Column 3 Worm Methane I I I 38 22 Gas YEEY': I D 22 II 24 7 lnfercoolers Steam y E 39 water 2 Liquid Rich LNG 29 1- 200 34 550*- 245 44 M Cooling Water 3| 35 Rich LNG Leon Gas 32 200F. 550'=H-Am biem 5505* Temp.

33 mos Ambien? Temp INVENTOR A Iexonder Harmens XAW ATTORNEY July 19, 1966 A. HARMENS 3,261,169

METHOD OF PROCESSING A MIXTURE OF LIQUEFIED GASES 11ed Oct 21, 1963 2 Sheets-Sheet 2 eon as 43 FIG. 2

Condenser.

Leon Gci L -4o0 1 .i. i Vcponzerw '8 Liquid Etha 39 J9 V 3 550* Z 62\ 7o 5 E pansion Engine Dynamo Separator I6 53 Eihune Heater For a 3 Fluid In 5| +800 h 69 56 Fractionation 67 Hem Column Sfeom I i 57 Exch. 6| quid G05 66 35 5? 29 2B 550a- E h 59 24 lg. t Sieum n92 72= 64 6O Line Liquid 34 27 Rich LNG Cooling 30 Water Rich LNG /33 ecin Gas now 55.0% AmbienfTemp. Amb'em Temp INVENTOR A Iexonder Hcirmens BY 4 1% r ATTORNEY United States Patent 4/63 4 Claims. (Cl. 62-28) This invention concerns improvements in or relating to the processing of a mixture of liquefied gases. More particularly, it concerns a method of separating liquefied natural gas into a gaseous product and a liquid product, the one consisting predominantly of methane and ethane and the other of higher hydrocarbons, and being hereafter referred to as a lean gas and a rich liquefied gas respectively.

Natural gas is a gas occurring naturally in underground accumulations, usually associated with oil deposits, and when extracted, and purified if necessary, consists almost entirely of volatile hydrocarbons. Natural gas can be liquefied to facilitate its storage and transport, liquefaction reducing the volume of unit mass of the gas to about of its value under atmospheric pressure. The liquefied natural gas can be partly evaporated to produce a lean gas and a rich liquefied gas. An object of the invention is to provide an efiicient method of carrying out this separation.

According to the invention, a method of separating liquefied natural gas into a lean gas and a rich liquefied gas comprises:

(a) Partly vaporising the liquefied natural gas to separate some lean gas therefrom;

(b) Subjecting residual liquid from (a) to fractionation to give a gaseous top product and a liquid bottom product;

(c) Partly condensing said gaseous top product to obtain further lean gas;

(d) Recycling the condensate from (c) to give a descending liquid stream in said fractionation;

(e) Partly vaporising said liquid bottom product to obtain rich liquefied gas; and

(f) Recycling gas from (e) to give an ascending gas stream in said fractionation.

The part vaporisation of liquefied natural gas in step (a) can be effected under a pressure above, below or equal to that under which residual liquid is fractionated in step (b) according to the economic and other circumstances of the operation. The pressure in step (b) is between a pressure low enough to facilitate fractionation and a pressure high enough to enable the invention to provide lean gas at a convenient pressure for transport.

Ifdesired, cold in the liquefied natural gas available can be utilised. Thus, if it is desired to use the vaporising liquefied natural gas in step (a) as a source of cold below the temperature of the liquefied natural gas entering the system, the vaporisation should be effected under a pressure lower than that at which the liquefied natural gas is available. The resultant lean gas from step (a) can then be compressed to a pressure suitable for subsequent use of the lean gas. If there is no use for such a source of cold, the pressure on the liquefied natural gas available can be increased to the pressure required in the fractionation in step (b), and the resultant liquefied natural gas, before being partly vaporised in step (a), can be used to cool by indirect heat exchange the working medium of a closed compression/ expansion system for converting heat into mechanical power.

In either event, before subjecting residual liquid from step (a) to fractionation in step (b), the residual liquid can be passed in indirect heat exchange with a mixture of lean gas from step (c) and lean gas from step (a), or lean gas from step (a) that has been compressed, to cool said mixture. Also, the residual liquid from step (a), immediately before being subjected to fractionation in step (b), can be used as a separate coolant in the part condensation of gaseous top product in step (0).

At least part of a mixture of lean gas obtainable by mixing lean gas from step (c) with lean gas from step (a), or lean gas from step (a) that has been compressed, can be used as a separate coolant in the part condensation of gaseous top product in step (c). The lean gas thus used as coolant can be passed out of the system together with any lean gas that was not used as coolant. If desired, immediately before this mixture leaves the system, it can be warmed by indirect heat exchange with rich liquefied gas from step (f). At least part of the rich liquefied gas used to Warm said mixture of lean gas can then be used to warm by indirect heat exchange the working medium of a closed compression/expansion system for converting heat into mechanical power.

The various steps described above effect a sharp separation of the lean gas and rich liquefied gas. The lean gas produced can have a higher heat of combustion per unit mass than the gas obtained by completely vaporising the liquefied natural gas feedstock. The lean gas can be utilised in, for example, gas burners that might otherwise produce incomplete burning and become foul if gas obtained by completely vaporising the liquefied natural gas feedstock were burned. The rich liquefied gas obtained can be marketed as liquefied petroleum gas.

The invention will now be illustrated by the following examples, in which reference is made to the accompanying drawings. In the drawings, FIGURES 1 and 2 are flow diagrams for practising the invention.

Example I Liquefied natural gas under a pressure of 14.7 p.s.i.a. and at a temperature of 245 F. consisting of methane, 55 mole percent; ethane, 22 mole percent; propane, 16 mole percent and n-butane, 7 mole percent enters the system at a constant rate via a liquid line 1 and passes through a reduction valve 2 into a liquid line 3. During the passage of liquefied natural gas through valve 2, some of the liquefied natural gas is converted into flash vapour. The liquefied natural gas and flash vapour in line 3 are under a pressure of 3 p.s.i.a. and pass into the cold side of a vaporisation vessel 4. In the cold side of vessel 4, the liquefied natural gas vaporises under equilibrium conditions to produce a lean gas substantially consisting of methane and a residual liquid consisting of methane, 20 mole percent; ethane, 39 mole percent; propane, 29 mole percent and n-butane, 12 mole percent. The necessary heat for producing the vaporisation of the liquefied natural gas is provided by methane in the warm side of vessel 4 obtained under a pressure of 22 p.s.i.a. from a gas line 5. The methane in the warm side of vessel 4 is thereby cooled to a temperature of 250 F. and condenses; the resultant liquefied methane leaves the warm side of vessel 4 via a liquid line 6. The liquefied methane can be used outside the system as a low temperature coolant.

Residual liquid at a temperature of -260 F. is pumped from the cold side of vessel 4 into a liquid line 7 by a pump 8, and then passes via pump 8, a liquid line 9, the cold side of three intercoolers 10 connected in series, a liquid line 11, the cold side of a heat exchanger 12, a liquid line 13, a separate part of the cold side of a condenser 14 and a gas/liquid line 15 into a fractionation column 16. The residual liquid, and vapour thereof, enters column 16 under a pressure of 550 p.s.i.a. The temperatures of the residual liquid leaving the third intercooler 10, heat ex- 3 changer 12 and condenser 14 are respectively -195 F., 120" F. and 60 F.

In fractionation column 16, fractional distillation of the residual liquid, and vapour thereof, occurs to produce as top product a lean gas consisting of methane, 19 mole percent; ethane, 72 mole percent; propane, 7 mole percent and n-butane, 2 mole percent and as bottom product a liquid consisting of ethane, 12 mole percent; propane, 65 mole percent and n-butane, 23 mole percent.

The top product passes from fractionation column 16 via a gas line 17 into the warm side of condenser 14, in which it is partly condensed to produce a mixture of a lean gas consisting of methane, 35 mole percent; ethane 61 mole percent and propane, 4 mole percent and a liquid condensate consisting of methane, 13 mole percent; ethane, 75 mole percent; propane, 9 mole percent; and n-butane, 3 mole percent. The lean gas and liquid condensate pass from the warm side of condenser 14 via a gas/liquid line 18 into a separator 19. In separator 19, the lean gas separates from the liquid condensate. The separated lean gas passes under a pressure of 550 p.s.i.a. and at a temperature of 33 F. into a gas line 20 connected to the gas outlet of the separator. The separated liquid condensate in separator 19 is recycled via a liquid line 21 into fractionation column 16 to produce a descending liquid stream therein.

The liquid bottom product passes from fractionation column 16 via a liquid line 22 into the cold side of a reboiler 23, in which it partly vaporises to produce a gas and a rich liquefied gas. The necessary heat for producing the vaporisation of the bottom product is provided by waste steam condensing in the warm side of the reboiler obtained under a pressure of 14.7 p.s.i.a. and at a temperature of 212 F. from a gas line 24. Aqueous condensate leaves the warm side of the reboiler via a liquid line 25.

The gas produced in the cold side of the reboiler 23 is recycled via a gas line 26 into the bottom of fractionation column 16 to provide an ascending gas stream therein.

Rich liquefied gas consisting of ethane, 8.3 mole percent; propane, 63 mole percent; and n-butane, 28.7 mole percent under a pressure of 550 p.s.i.a. and at a temperature of about 200 F. passes from reboiler 23 via a liquid line 27, the warm side of a heat exchanger 28, a liquid line 29, the warm side of a further heat exchanger 30, a liquid line 31, a reduction valve 32 and a liquid line 33 out of the system as product under a pressure of 110 p.s.i.a. and at ambient temperature. In heat exchanger 28, the rich liquefied gas warms by indirect heat exchange lean gas in the cold side of the heat exchanger as described hereafter. In heat exchanger 30, the rich liquefied gas is cooled by indirect heat exchange with cooling water in the cold side of the heat exchanger, the cooling water entering the cold side of the heat exchanger via a liquid line 34 and leaving via a liquid line 35.

Lean gas at a temperature of -260 F. passes from vaporisation vessel 4 via a gas line 36, four compressors 37 connected in series and the cold sides of three intercoolers fitted between the compressors into a gas line 38. In line 38, the lean gas mixes with lean gas from incoming branch line 20. The resultant mixture of lean gas passes through the warm side of heat exchanger 12 into a gas line 39. Part of the lean gas then passes via an outgoing branch line 40 of line 39, a further separate part of the cold side of condenser 14 and an incoming branch line 41 of line 39 fitted with a control valve 42 into line 39. The remainder of the lean gas continuing along line 39 passes through a control valve 43 fitted in line 39 and then mixes with lean gas from incoming branch line 41. The resultant mixture of lean gas in line 39 passes into the cold side of heat exchanger 28, in which its temperature is raised to ambient temperature by indirect heat exchange with rich liquefied gas in the Warm side of the heat exchanger. The lean gas then passes out of the system via a gas line 44 as product under a pressure of 550 p.s.i.a. and at ambient temperature.

Example ll Liquefied natural gas under a pressure of 14.7 p.s.i.a. and at a temperature of 245" F. consisting of methane, mole percent; ethane, 22 mole per-cent; propane, 16 mole percent and n-butane, 7 mole percent enters the system via a liquid line 50 and is pumped under a pressure of 550 p.s.i.a. into a liquid line 51 by a pump 52. The liquified natural gas then passes via the cold side of a heat exchanger 53 and liquid line 3 into the cold side of vaporisation vessel 4. In the cold side of vessel 4, the liquified natural gas vaporises under equilibrium conditions to produce a lean gas consisting of methrane, 87 mole percent; ethane, 10 mole percent and propane, 3 mole percent and a residual liquid consisting of methane, 38 mole percent; ethane, 28 mole percent; propane, 23 mole percent and n-butane, 11 mole percent.

Residual liquid at a temperature of 40 F. passes from the warm side of vaporisation vessel 4 via a liquid line 13, a separate part of the cold side of condenser 14 and a gas/liquid line 15 into fractionation column 16 under a pressure of 550 p.s.i.a.

In fractionation column 16, fractional distillation of the residual liquid, and vapour thereof, occurs to produce as top product a lean gas consisting of methane, 39 mole percent; ethane, 59 mole percent and propane, 1.5 mole percent and n-butane, 0.5 mole percent and as bottom product .a liquid consisting of ethane, 12 mole percent; propane, 65 mole percent and n-butane, 23 mole percent.

The top product passes from fractionation column 16 via gas line 17 into the warm side of condenser 14, in which it is partly condensed. A mixture of a lean gas and a liquid condensate passes from the warm side of condenser 14 via a gas/ liquid line 54 into the cold side of vaporisation vessel 4, in which it provides the necessary heat for vaporising the liquefied natural gas in the warm side of vessel 4. The mixture of lean gas and liquid condensate in the Warm side of vessel 4 is thereby cooled and further condensed to produce a mixture of lean gas consisting of methane, mole percent; ethane, 39 mole percent; and propane, 1 mole percent and a liquid condensate consisting of methane, 24 mole percent; ethane, 73 mole percent; propane, 2 mole percent and n-butane, 1 mole percent. This lean gas and liquid condensate pass from the warm side of vessel 4 via gas/ liquid line 18 into separator 19. In separator 19, the lean gas separates from the liquid condensate. The lean gas passes under a pressure of 550 p.s.i.a. and at a temperature of '15 F. into gas line 20 connected to the gas outlet of separator 19. The liquid condensate in separator 19 is recycled via liquid line 21 into fractionation column 16.

The liquid bottom product passes from fractionation column 16 via liquid line 22 into the cold side of reboiler 23 in which it vaporises to produce a gas and a rich liquefied gas. The necessary heat for vaporising the bottom product is provided by waste steam. condensing in the warm side of the reboiler obtained under a pressure of 14.7 p.s.i.a. and at a tempem'aturre of 212 F. from gas line 24.- Aqueous condensate leaves the warm side of the reboiler via (liquid line 25.

Rich liquefied gas as product consisting of ethane, 8.3 mole percent; propane, 63 mole percent and n-butane, 28.7 mole percent under a pressure of 550 p.s.i.a. and at a temperature of about 200 F. passes out of the system from reboiler 23 in the following manner.

The rich liquefied gas passes from reb oiler 23 via liquid line 27 and the warm side of heat exchanger 28 into liquid line 29. In heat exchanger 28, the rich liquefied gas warms 'by indirect heat exchange lean gas in the cold side of the heat exchanger as described hereafter. Part of the rich liquefied gas then passes via an outgoing bnanoh (line 55 of line 29 fitbed With a control valve 56, a separate part of the cold side of "a heat exchanger 57, a liquid line 58, the Warm side of a further heat exchanger 59 and an incoming bmanch line 60 of line 29 into line 29. The remainder of the rich liquefied gas continuing along line 29 passes through a control valve 61 fitted in line 29 and then mixes with the rich liquefied gas from incoming branch line 60. The resultant mixture of rich liquefied gas passes into the Warm side of heat exchanger 30, in which it is cooled to ambient temperature by indirect heat exchange with cooling Water in the Cold side of the heat exchanger, the cooling water entering the cold side of the heat exchanger via liquid line 34 and leaving via liquid line 35. The rich liquefied gas then passes via liquid line 31, reduction valve 32 and liquid line 33 out of the system as product under a pressure of 110 p.s.i.a. and at ambient temperature.

Lean gas at a temperature of 40 F. passes from vaporisation vessel 4 into gas line 39, in which it mixes with lean gas from incoming branch line 20. Part of the mixture of lean gas passes via an outgoing branch line 40 of line 39, a separate part of the cold side of condenser 14 and an incoming branch line 41 of line 39 fitted with control valve 42 into line 39. The remainder of the lean gas continuing along the line 39 passes through control valve 43 fitted in line 39 and then mixes with lean gas from incoming branch line 41. The resultant mixture of lean gas passes into the cold side of heat exchanger 28, in which its temperature is naised to ambient temperature by indirect heat exchange with the rich liquefied gas in the Warm side of heat exchanger 2.3. The lean gas then passes out of the system via gas line 44 as product under a pressure of 550 p=.s.i.a. and at ambient temperature.

The warm side of heat exchanger 53 is the cold element of a closed compression/expansion system for converting heat into mechanical power. The system enables cold in the liquefied natural gas passing through the cold side of heat exchanger 53 to be used in a productive way. The system is operated in the following manner. Lique fied natural gas in the cold side of heat exchanger 53 cools and condenses by indirect heat exchange ethane (or any other suitable working medium) in the warm side of the heat exchanger obtained under a pressure of 60 p.s.i.a. and at a tempenature of 72 F. from a gas line 62. Liquefied ethane at a temperature of 72 F. is pumped under a pressure of 1,190 p.s.i.a. from the warm side of heat exchanger 53 via a liquid line 63 into a liquid line 64 by a pump 65. The liquefied ethane then enters the cold side of heat exchanger 59, in which its tempeuatutre is raised to about 35 F. by indirect heat exchange with the rich liquefied gas in the warm side of the heat exchanger. Liquefied ethane passes from the cold side of heat exchanger 59 via a liquid line 66 into a separate part of the cold side of further heat exchanger 57. Heat exchanger 59 acts as a pie-warmer for the liquefied ethane, and ensures that the liquefied ethane entering heat exchanger 57 does not produce such a cooling therein as to cause formation of ice in the warm side of heat exchanger 57. In the cold side of heat exchanger 57, the liquefied ethane is vaporised by indirect heat exchange with waste steam condensing in the warm side of the heat exchanger obtained under a pressure of 14.7 p.s.i.a. and at a temperature of 212 F. from a gas [line 67. The waste steam also wanms by indirect heat exchange the rich liquefied gas in the other part of the cold side of heat exchanger 57, from which the Warmer crich liquefied gas passes via line 58 into the warm side of heat exchanger 59. Aqueous condensate leaves heat exchanger 57 via a 'gas line 68. Gaseous ethane under a pressure of 1,190 p.s.i.a. and at a temperature of about 200 F. passes from the cold side of heat exchanger 57 via a gas line 69 into an expansion engine 70 for producing mechan ical power by reducing the pressure on the gaseous ethane to 60 p.s.i.a. Expansion engine 7 is coupled to a dynamo 71 for generating electricity. Ethane passes from expansion engine 70 via gas line 62 into the warm side of heat exchanger 53.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined in the appended claims.

I claim:

1. A method of separating liquefied natural gas into a lean gas and a rich liquefied gas, comprising:

(a) partly vaporising the liquefied natural gas to separate some lean gas therefrom;

(b) subjecting residual liquid from (a) to fractionation to give a gaseous top product and a liquid bottom product;

(c) partly condensing said gaseous top product to obtain further lean gas;

((1) recycling the condensate from (c) to give a descending liquid stream in said fractionation;

(e) partly vaporising said liquid bottom product to obtain rich liquefied gas;

(f) recycling gas from (e) to give an ascending gas stream in said fractionation;

(g) increasing the pressure of the liquefied natural gas to the pressure required in the fractionation in step (b) before it is partly vaporised in step (a);

(h) before partly vaporising the liquefied natural gas in step (a), using the liquefied natural gas to cool by indirect heat exchange a separate gas supply used as the working medium of a closed compression/expansion system for converting heat into mechanical power by expanding the cooled working medium in an expansion engine.

2. A method as claimed in claim 1 in which said working medium is methane.

3. A method of separating liquefied natural gas into a lean gas and a rich liquefied gas, comprising:

(a) partly vaporising the liquefied natural gas to separate some lean gas therefrom;

(b) subjecting residual liquid from (a) to fractionation to give a gaseous top product and a liquid bottom product;

(c) partly condensing said gaseous top product to obtain further lean gas;

((1) recycling the condensate from (c) to give a descending liquid stream in said fractionation;

(e) partly vaporising said liquid bottom product to obtain rich liquefied gas;

(f) recycling gas from (e) to give an ascending gas stream in said fractionation;

(g) the step of mixing lean gas from step (c) with lean gas from step (a);

(h) using said mixture of lean gas as a separate coolant in the part condensation of gaseous top product in step (c);

(i) immediately before passing the mixture of the lean gas out of the system, warming said mixture by in direct heat exchange with rich liquefied gas from step (e); and

(j) after the said residual liquid from step (e) has warmed the mixture of lean gas, using at least part of said residual liquid to warm by indirect heat exchange a liquid gas used as the working medium of a closed compression/expansion system for converting heat into mechanical power.

4. A method as claimed in claim 3, in which said working medium is ethane.

References Cited by the Examiner UNITED STATES PATENTS 2,342,165 2/ 1944 Plummer 62-27 X 2,471,602 5/ 1949 Arnold 6227 2,603,310 7/1952 Gilmore. 2,953,905 9/1960 Chrones 6228 2,973,834 3/1961 Cicalese.

NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, J. C. JOHNSON, Assistant Examiners. 

